Circuit breaker panels in most households and other buildings typically include a box-like enclosure with a regular array of breakers, for example arrayed in two parallel columns. The enclosure is typically protected with a door that can be latched shut. A printed reference chart or key is typically located on the inside surface of the door, organized with individual labels corresponding in number and position to the array of breakers (e.g., two parallel columns) so that a person servicing the panel, checking for tripped breakers, or disabling circuits prior to making electrical repairs elsewhere in the building can identify which breakers control which circuits or appliances.
Anyone who has ever tried to decipher the scribblings of successive homeowners or electricians on the reference chart will appreciate that the circuit labels are often difficult to read, especially in the cramped confines and poor lighting of the typical breaker panel location in a basement, utility closet, garage corner, etc. This difficulty is made worse if the circuit diagnosis is being made during a power outage or emergency, especially if by someone unfamiliar with the house or the particular panel.
It can also be difficult to make back-and-forth visual correspondence between the reference chart on the inside of the open door and the rows and columns of breakers on the opposing panel.
Even if the breaker reference chart is reasonably legible (a rarity) and well lit, the breakers themselves are normally not labeled, and visual correspondence must often be based on a number faintly stamped into the metal of the breaker panel next to the breaker.